In order to conserve bandwidth, most modern communication channels employ filters that aggressively eliminate waveforms that are unlikely to be human speech, or are unlikely to be detected by the human ear (the human ear has a hearing range between 20 and 20,000 Hz). Thus encrypted content that is sent directly over such a channel will be dramatically altered or eliminated by these filters.
In view of the foregoing, it would be desirable to form a voice encrypted signal in the hearing range to avoid digital filters used in packet switched networks.